Lithium secondary batteries have a higher energy density in theory compared to other batteries and thus allow to manufacture a small and light-weight battery. Therefore, vigorous studies have been focussed thereon to develop a power source of portable electronic instruments. Particularly, performance of such instruments is even increasing in recent years and their power source is required concominantly therewith to exhibit better discharging characteristics even at a high load. In order to fulfill these requirements, various studies are in progress next to the prior art battery using nonaqueous electrolyte solutions referred to as lithium ion battery to develop a battery using a polymer electrolyte that functions both as the nonaqueous electrolyte solution and the polymer separator of the prior art battery. Much interest has been focussed to a lithium secondary battery using the polymer electrolyte because of its remarkable advantages such as the possibility of making the battery smaller and thinner in size and lighter in weight as well as leak free.
Generally, secondary batteries now available in the market such as lithium secondary batteries make use of a nonaqueous electrolyte solution prepared by dissolving an electrolyte salt in an organic solvent. The use of this solution is problematic because the solution is easily susceptible to leakage from the battery parts, dissolution of electrode substances or vaporization which may develop problems of long term reliability, spilling off in the sealing process and the like.
In order to improve these problems, lithium secondary batteries have been developed which make use of a polymer electrolyte macroscopically occurring as a solid. The polymer electrolyte consists of a porous matrix of an ion-conductive polymer impregnated with or retaining a nonaqueous electrolyte solution (a lithium salt solution in an aprotic polar organic solvent).
Microscopically the polymer electrolyte has a continuous phase of nonaqueous electrolyte solution therein and exhibits a high ion conductivity. This results in a low mechanical strength. The mechanical strength may be reinforced by including a separator (porous substrate) in the polymer electrode but another problem still remains to exsist.
The lithium secondary battery relies on intercalation or doping of lithium into an electroactive substance which results in expansion and shrinkage of the electroactive layer. If the polymer electrolyte fails to accommodate well the expansion/shrinkage, then physical contact between the electrode and the polymer electrolyte will become unsatisfactory to develop increased interfacial resistance therebetween. This adversely affect the battery perfomance including discharge and charge cycle characteristics of the battery.
JP-A-5012913 discloses that the ion conductivity and the elasticity of the polymer electrolyte of this type may well be balanced by increasing the ratio of nonaqueous electrolyte solution to ion-conductive polymer to 200% or higher and also increasing the elasticity and elongation of the polymer electrolyte greater than certain levels. Since greater elasticity levels mean greater strain per unit amount of stress, increased elasticity cannot accommodate both of expansion and shrinkage. In order to accommodate both expansion and shrinkage, it is necessary for the polymer electrolyte to have a cushon-like property.
Accordingly, the problem to be solved by the present invention is to provide a lithium secondary battery including a polymer electrolyte layer that can tolerate expansion and shrinkage of the electroactive substance layers and exhibit a buffering effect as a whole.